The present invention relates to a guide device for use in, for example, an arm of an industrial robot and, more particularly, to a guide device for guiding an object which performs a composite motion composed of axial linear movement and rotational movement.
A known guide device for composite motion incorporate a shaft which is provided in the outer peripheral surface thereof with ball screw grooves and ball spline grooves which are formed on the same portion of the shaft in an overlapping manner, thereby to provide a large stroke of the composite motion. (See, for example, Japanese Patent Laid-open Publication No. 62-165057) More specifically, this type of known guide device has a nut mounted on the shaft through the intermediary of balls which are received in and guided by the ball screw grooves, and an outer spline sleeve mounted on the shaft through the intermediary of balls received in and guided by the ball spline grooves. The nut and the outer spline sleeve are received in a common housing. Each of the nut and the outer spline sleeve is rotatably supported in the housing through a pair of support bearing. Thus, the nut and the outer spline sleeve are independently rotatable. In operation, the nut and the outer spline sleeve are selectively rotated so as to enable the shaft to move axially, to rotate or to perform a composite motion composed of axial motion component and rotational motion component.
Problems encountered with this known device can broadly be sorted into three categories (A), (B) and (C).
(A) There exist inevitable play or looseness of the spline outer sleeve and the ball screw nut. Such looseness causes a phase delay particularly when the apparatus is started, and impairs the precision of stopping position.
(1) Referring first to the spline outer sleeve, critical looseness exists in the direction of rotation, because the rotational position is determined by the outer spline sleeve, while the ball screw nut is designed to rotate freely.
In the guide device of the type which has ball screw grooves and ball spline grooves in an overlapping manner, the shaft has to have a circular cross-section to enable the ball screw nut to rotate thereon through the balls which roll along the ball screw grooves formed in the outer peripheral surface of the shaft. On the other hand, the ball spline grooves are designed as radial ball spline as specified by Japanese Industrial Standards as shown in FIG. 14. Thus, the guide device as a whole is not designed to provide a rigid support against load which acts in the angular direction, though is serves satisfactorily to bear radial load. For these reasons, it has been difficult to satisfactorily eliminate any play in the direction of rotation.
(2) Referring now to the ball screw nut, the criticalness of the play exists in the axial direction, because the ball screw nut is required to regulate the position in the axial direction, while the spline outer sleeve is allowed to freely slide in the axial direction.
In the device of the type described, a moment of force is imparted by the shaft to the ball screw nut such as to rotate or twist the ball screw nut. This essentially requires that the nut is stably supported at its outer peripheral surface by a radial contact type bearing. This type of bearing, however, is not so rigid against any axial load, so that the guide device as a whole inevitably showed a play or looseness in the axial direction.
(B) The second problem is that fletching corrosion tends to be caused in the ball rolling surfaces due to the presence of play or looseness of the spline outer sleeve and the ball screw nut. The term "fletching corrosion" is used in this specification to mean wear of one or both of mutually contacting surfaces due to repeated minute relative sliding between these surfaces.
(1) A discription will be made hereinunder as to the fletching corrosion on the ball screw nut attributable to looseness of fit of the spline outer sleeve. When the rotating shaft is stopped abruptly, the shaft is displaced in the rotational direction due to inertia, because the balls supporting the spline outer sleeve are elastically deformed. When the elastically deformed balls rebound, the shaft is forced back in the rotational direction, whereby a vibration of the shaft takes place in the rotational direction. This vibration causes a minute vibratory slide between the balls and the ball rolling surfaces of the ball screw nut, resulting in local fletching corrosion of the ball rolling surfaces. As a result, local minute pits are formed in these ball rolling surfaces, thereby impairing smooth rolling of the balls.
(2) Fletching corrosion on the ball spline outer sleeve, attributable to axial play of the ball screw nut, takes place as follows.
When the axially running shaft is stopped abruptly, the shaft is displaced in the axial direction due to inertia, because the balls supporting the ball screw nut or balls of bearings are elastically deformed. When the elastically deformed balls rebound, the shaft is forced back in the axial direction, whereby a vibration of the shaft takes place in the axial direction. This vibration causes a minute vibratory slide between the balls and the ball rolling surfaces of the spline outer sleeve, resulting in local fletching corrosion of the ball rolling surfaces.
(C) The third problem is that the balls supporting the ball screw nut tend to be demaged due to the presence of radial play of the spline outer sleeve.
(1) When the guide device of this type is used in an industrial robot arm, the robot arm is usually fixed to the spline outer sleeve, so that the radial load is borne by the spline outer sleeve.
When a heavy lateral load is applied to the end of the shaft when the latter is in the extended state, an excessively heavy load is applied to the spline outer sleeve, resulting in a radial rattle of the spline outer sleeve. This radial rattle of the spline outer sleeve causes the shaft to oscillate in such a manner as to "twist" or "skew" the ball screw nut which fits in the same housing. In consequence, balls in both axial ends of the nut are overloaded to be elastically deformed, allowing a vibration to occur and to impair smooth circulation of the balls. In the worst case, the balls are damaged seriously.
(2) In the case where a play or looseness of the spline outer sleeve exists, the shaft minutely vibrates when the machine, e.g., a robot, is abruptly stopped, with the result that fletching corrosion is caused in the ball rolling surfaces on the ball screw nut.
(3) It is also to be understood that any radial play of the spline outer is amplified at the free end of the shaft, resulting in a degradation of the precision.